You're not officially supposed to listen to Christmas music until after Thanksgiving, but Pentatonix's a cappella cover of "Little Drummer Boy" is worth breaking the rules for.
The five-person singing group dubbed PTX — comprised of Avi Kaplan, Scott Hoying, Kirstin Maldonado, Kevin Olusola and Mitch Grassi — formed in 2011 in Arlington, Texas. When they competed in the NBC a cappella competition show "The Sing Off," they won, landing a recording contract with Sony Music and $200,000.
The group released its "PTXmas" album just in time for the holidays, but they found their true audience on YouTube. With close to 16 million subscribers and 3.5 billion views, Pentatonix's YouTube channel is one of the YouTube's most popular channels. They've released more music through Madison Gate Records, a label owned by Sony Pictures, and even made an appearance in "Pitch Perfect 2."
Editor's note: This story was originally published in 2013 and has been updated with new information.